A general box making machine manufactures boxes (corrugated board boxes) by processing sheet materials (for example, corrugated board sheets), and includes a paper feeding unit, a printing unit, a slotter creaser, a folding unit, and a counter ejector. The paper feeding unit sends out corrugated board sheets stacked on a table one by one, and feeds them to the printing unit at a constant speed. The printing unit includes a print unit, and prints images on the corrugated board sheets. The slotter creaser performs processing of forming ruled lines as folding lines, grooves as flaps, and glue application tabs for jointing ends on the corrugated board sheets on which images are printed. The folding unit applies glue to the glue application tabs and folds, along the ruled lines, the corrugated board sheets being moved on which ruled lines, grooves, and glue application tabs are formed, and joints the glue application tabs to manufacture flat corrugated board boxes. The counter ejector stacks the corrugated board boxes formed by folding the corrugated board sheets and applying glue, sorts the corrugated board boxes into a predetermined number of batches and discharges them.
A corrugated board box manufactured by the box making machine turns out to be a distorted nonconforming product when a corrugated board box is assembled into a case with its joint not jointed to achieve a gap of a predetermined dimension between adjacent flaps. A conventional technique of inspecting conformity of a joint by detecting a dimension between edges of flaps of the joint includes the one disclosed in Patent Document 1.    [Patent Document 1] Japanese Patent No. 2843783